PROJECT SUMMARY/ABSTRACT Tobacco smoking among women in Romania increased dramatically, by close to 30%, between 2011-2018 and the share of smokers with quit attempts decreased by 37%. Tobacco kills more than 8 million people a year around the world. The burden of tobacco-related illness and death is heaviest in low- and middle-income countries (LMICs) like Romania, where 80% of the worldwide 1.1 billion smokers live. Our work showed that 30% of women smoke pre-pregnancy in Romania and half of the smokers continue during pregnancy. It is critically important to address tobacco smoking during pregnancy, an extraordinary window of opportunity for sustained cessation. The objectives of this application are to develop, implement, and test the feasibility and effectiveness of an adaptive mobile pregnancy tobacco cessation app-based intervention using deep tailoring and a self-nominated support person, and to build mHealth research capacity in Romania. We build on our pregnancy smoking cessation trials with spousal support in Romania, including the ongoing Smoke Free Together (SFT1.0) mHealth pilot trial and the Quit Together telephone counseling trial that shows preliminary evidence of efficacy, and on our partnerships in Romania. The central hypothesis is that the intervention will show evidence of feasibility and effectiveness in increasing positive support, pregnancy cessation, and postnatal abstinence. The app will be novel in its use of the unique functionality of smartphones, use of reinforcement learning (RL) and deep tailoring to continuously adapt the intervention, the emphasis on increasing positive support, and the use of the app by both smoker and quit-partner. The aims are: 1. (R21 phase). Develop and test the feasibility and acceptability of the SFT2.0 app-based mobile smoking cessation intervention with quit-partner support during pregnancy and postpartum in Romania; 2. (R33 phase). Test the SFT2.0 app-based smoking cessation intervention in a hybrid effectiveness and implementation randomized controlled trial; 3. (R21 and R33 phases). Develop mHealth research capacity by enhancing individual and institutional research capabilities in Romania and expanding the existing international research network. With respect to expected outcomes, the proposed application is anticipated to advance understanding of the effectiveness and implementation of a smartphone-based app for pregnancy smoking cessation with quit- partner support. Such results are anticipated to have a system wide sustainable positive impact on the prevention of prenatal and postnatal smoking as the intervention has a high potential to be adopted in the universal health system in Romania, with applications to the US and other populations. The development of the international research network and the individual and institutional mHealth research capacity building is expected to positively impact mHealth research in CEE and the neighboring region.